Tar sands which are also known as oil sands and bituminous sands are siliceous materials which are impregnated with a heavy petroleum. The largest and most important deposits of the sands are the Athabasca sands, found in northern Alberta, Canada. These sands underlay more than 13,000 square miles at a depth of 0 to 2000 feet. Total recoverable reserves after extraction and processing are estimated at more than 300 billion barrels--just equal to the world-wide reserves of conventional oil. The tar sands are primarily silica, having closely associated therewith an oil film which varies from about 5 to 21 percent by weight, with a typical amount of 13 weight percent of the sand. The oil is quite viscous--6.degree. to 8.degree. API gravity--and contains typically 4.5 percent sulfur and 38 percent aromatics. The sands contain, in addition to the oil and sand components, clay and silt in quantities of from 1 to 50 weight percent, more usually 10 to 30 percent. The sands also contain a small amount of water, in quantities of 1 to 10 percent by weight, in the form of a capsule around the sand grains.
Several basic extraction methods have been known for many years for the separation of oil from the sands. The most important method which now has been proven commercially feasible is the hot water extraction process currently in use at Great Canadian Oil Sands, Ltd. plant located 20 miles north of Fort McMurray, Alberta, Canada.
One mode of the hot water process is disclosed in Canadian Pat. No. 841,581 issued May 12, 1970 to Floyd et al. This process provides that the bituminous sands are jetted with steam and mulled with a minor amount of hot water at temperatures of 170.degree. to 190.degree.F., and the resulting pulp is then dropped into a turbulent stream of circulating hot water and carried to a separation cell maintained at a temperature of about 185.degree.F. In the separation cell, sand settles to the bottom as tailings and oil rises to the top in the form of a froth. An aqueous middlings layer comprising clay and silt and about 1 to 5 weight percent bitumen based on the weight of the middlings is formed between these layers. This basic process may be combined with a scavenger step for further treatment of the middlings layer obtained from the primary separation step to recover an additional amount of oil therefrom.
The middlings layer withdrawn from the hot water separation cell in a hot water extraction process contains most of the silt and clay as well as some bitumen. In the hot water extraction process disclosed by Floyd et al. above, a stream of middlings is withdrawn from the primary extraction zone and recycled to the conditioning vessel of the hot water process. Also, a second stream of the middlings is transferred from the primary extraction zone to an air scavenger zone wherein air is bubbled into the material in the scavenger zone to cause flotation of additional bitumen from the middlings material. This bitumen is then recovered as a froth and combined with the bitumen froth recovered from the primary extraction zone. The depleted middlings stream is normally thereafter discarded into a retention pond or in some circumstances combined with the sand tailings layer which was removed from the primary extraction zone and subsequently discarded.
Canadian Pat. No. 882,667 issued Oct. 5, 1971 to Erskine et al. provides a hot water process separation cell of the type which is suitable for use in a process as disclosed by Floyd et al. above. Essentially, this cell is a circular tank having a convex bottom with a sand tailings exit means at the center thereof, middlings outlets on the side walls of the cell, a center rotating shaft fitted with sand rakes on the bottom end, and froth skimmers situated on the upper end. The cell also has an overflow launder on the top of the tank walls to provide a means of recovering the bitumen froth product of the process.
Copending U.S. application Ser. No. 516,063 filed of even date herewith discloses an improvement in the hot water process provided by Floyd et al. Essentially, the improvement comprises inserting water below the upper surface of the sand tailings layer formed in the bottom of a hot water separation cell when extracting bitumen from tar sands. In the prior art process, the intersticies of the sand tailings layer in the bottom of a hot water separation cell are filled with bitumen-rich middlings material containing 2 to 5 weight percent bitumen. When the sand tailings layer is withdrawn from the settling cell and discarded, the bitumen-rich middlings are also removed from the cell and this quantity of bitumen is lost from the recovery process.
By displacing the bitumen-rich middlings from the intersticies of the sand tailings layer with other water, the bitumen-rich middlings is backed out of the intersticies of the tailings layer and remains in the process stream so that the bitumen in the middlings can be recovered as product. This method of displacing bitumen-rich fluids from the intersticies of the sand tailings layer with a bitumen-lean fluid is often referred to as a hydroseparation method for recovering bitumen from tar sands or simply a "hydroseparation hot water process" and the displacing fluid as the hydroseparation liquid.
It has been discovered that unfortunately the settling cell provided by Erskine et al. above does not readily lend itself to the hydroseparation hot water process. This shortcoming is due in part to the design of the bottom of the cell. Thus, a need for a new and improved hot water separation cell suitable for use in a hydroseparation hot water process has become apparent. The apparatus of the present invention provides one means of filling that need.